The present invention relates to superconducting solenoid coils of the type which are maintained superconductive by being immersed in a cryogenic bath.
The unique electrical properties of superconductors have led to proposals for storing large quantities of electrical energy in large superconducting coils, one type of which would be an annular solenoid installed, possibly below ground, so that its axis is vertical. The proposed solenoids would have a substantial axial height and diameter, the diameter possibly being of the order of 500 to 1000 meters or more.
The superconductive materials which are currently usable in practice, such as NbTi, must be maintained at a temperature in the vicinity of 1.degree.-4.degree. K. in order to exhibit superconducting properties. Such temperatures can be established by surrounding the superconducting material with successive envelopes of cryogens having progressively higher boiling points.
For example, it is known to use, for this purpose, an envelope of helium, which can be made to have a boiling point lower than 4.degree. K., surrounded by an envelope of neon, having a boiling point of in the vicinity of 28.degree. K., the latter being surrounded by an envelope of nitrogen, having a boiling point in the vicinity of 77.degree. K. Each of these cryogens is maintained in the liquid state by a suitable refrigeration system.
During operation of such a solenoid at superconducting temperatures, the varying magnetic fields generated during operation will cause adjacent layers of the solenoid structure, and particularly the layers of insulating material, to slide relative to one another, producing heat due to friction. Sufficient heat can be generated to produce local hot spots which can bring the conductor above its superconducting temperature. This will result in further heat generation which can cause the entire coil to cease to be superconducting.